1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image formed by an image forming method such as electrophotography, electrostatic recording and electrostatic printing. In addition, the present invention also relates to a developer including a toner,.and a method for fixing a toner image.
2. Discussion of the Background
Electrophotographic image forming methods are widely used for copiers, facsimile machines, laser printers, etc. The electrophotographic image forming methods typically include the following processes:    (1) charging a photoreceptor (charging process);    (2) irradiating the photoreceptor with imagewise light to form an electrostatic latent image thereon (imagewise light irradiation process);    (3) developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor (developing process);    (4) transferring the toner image onto a receiving material such as paper optionally via an intermediate transfer medium (transfer process);    (5) fixing the toner image on the receiving material, for example, upon application of heat and pressure thereto (fixing process); and    (6) cleaning the surface of the photoreceptor (cleaning process).
In order to produce high quality image, it is important to faithfully develop an electrostatic latent image with a developer (i.e., a toner), and requisites for the toner are preservation property (blocking resistance), feeding ability, developing ability, transferring ability, charging ability, fixing ability, etc.
Methods for producing toners are broadly classified into two methods, pulverization methods and suspension polymerization methods.
Procedures of the pulverization methods are as follows:    (1) toner constituents such as thermoplastic resins, colorants, charge controlling agents and other additives are uniformly mixed and kneaded upon application of heat thereto;    (2) the kneaded mixture is cooled and then pulverized to prepare a colored powder; and    (3) the colored powder is classified to prepare toner particles.
Toners prepared by pulverization methods have a certain degree of properties. However, there are narrow options for selection of materials in the pulverization methods. For example, the kneaded mixture has to be pulverized by a general pulverizer and classified by a general classifier, i.e., the kneaded mixture has to be so brittle as to be easily pulverized. Therefore, when kneaded mixture is pulverized, the resultant colored powder has a broad particle diameter distribution.
In this case, in order to produce toner images having good resolution and gradation, fine particles having a particle diameter not greater than 5 μm and coarse particles having a particle diameter not less than 20 μm have to be removed in the classification process, resulting in occurrence of a problem in that the yield seriously decreases in the pulverization methods. In addition, it is difficult to uniformly disperse a colorant and a charge controlling agent in a thermoplastic resin. If such agents are non-uniformly charged in a binder resin, the fluidity, developing ability, and durability of the resultant toner, and the image qualities of the toner images deteriorate.
Recently, in order to settle the problems of the pulverization methods, polymerization methods have been proposed and practically performed. The technique for producing a toner using a polymerization method is well known. For example, suspension polymerization methods are used for preparing a toner. However, the toner prepared by a suspension polymerization method has a drawback of having a poor cleaning property because of having a spherical form.
When images having a low image area proportion are produced, the amount of toner particles remaining on a photoreceptor is small, and therefore the cleaning problem does not occur generally. However, when images, such as pictures, having a high image area proportion are produced or a large amount of toner particles accidentally remains on a photoreceptor (due to paper jamming, for example), a problem in that the resultant images have a background fouling occurs.
In addition, toner particles remaining on a photoreceptor contaminate the charging roller used for charging the photoreceptor, and thereby the charging ability of the charging roller is deteriorated.
In attempting to solve such problems, a technique in which resin particles prepared by an emulsion method are associated with each other to prepare a toner having an irregular form is proposed in Japanese patent No. 2,537,503. However, toner particles prepared by an emulsion method include a large amount of emulsifier thereon and therein even when the toner particles are washed. Therefore, the resultant toner has a poor environmental stability, and a broad charge quantity distribution, resulting in occurrence of background fouling in the resultant images. In addition, the surfactant remaining on the toner particles contaminates the photoreceptor, charging roller and developing roller used, and thereby the members lose their original functions.
In addition, the method in which a toner is prepared by associating resin particles prepared by an emulsion polymerization method has the following drawbacks:    (1) fine particles of a release agent, which are typically included in the toner to improve the offset resistance of the toner, are included inside of the toner particles, and thereby good offset resistance cannot be imparted to the toner;    (2) since resin particles, release agent particles and colorant particles are randomly adhered to each other to constitute toner particles, the resultant toner particles have variations in formula (i.e., variations in contents of the toner constituents) and molecular weight of the resin particles included therein, i.e., the toner particles have different surface properties, and thereby images having good image qualities cannot be stably produced; and    (3) in a low temperature fixing device, images of the resultant toner cannot be fixed at a relatively low fixing temperature because resin particles are mainly present on the surface of the toner particles.
When toner images are fixed while brought into contact with a heat roller, the toner has to have good releasability against the heat roller (i.e., the toner has to have good offset resistance). By including a release agent on the surface of toner particles, the offset resistance of the toner particles can be improved.
Published unexamined Japanese patent applications Nos. (hereinafter JOPs) 2000-292973 and 2000-292978 disclose that resin particles are not only included in the toner particles but also are unevenly present on the surface of the toner particles, to improve the offset resistance of the toner. However, the minimum fixable temperature of the toner increases, i.e., the toner has poor low temperature fixability or poor energy-saving fixability.
In attempting to avoid the offset problem, methods in which a release oil such as silicone oils is applied to the surface of a fixing roller have been typically used. The methods are useful for preventing occurrence of the offset problem, but it is necessary to provide an applicator applying such a release oil, resulting in jumboization of the fixing device and increase in costs of the fixing device.
Therefore, in the case of a monochrome toner, a technique in which the viscoelasticity of the toner is increased, for example, by controlling the molecular weight distribution of the resin included in the toner is used for preventing internal fracture of the toner melted by a heat roller, while adding a release agent such as waxes to the toner to improve the release property of the toner. Thus, fixing methods which use the technique and in which no oil or a small amount of oil is applied to a fixing roller are typically used now.
Recently, a strong need exists for energy-saving image forming apparatus such as copiers and printers. Therefore a need exists for a toner having a low temperature fixability. In order to improve the low temperature fixability of a toner is improved, the viscoelasticity of the toner has to be decreased when the toner is melted, resulting in occurrence of the offset problem. It is effective to decrease the glass transition temperature (Tg) of the binder resin of a toner when improving the low temperature fixability of the toner. In this case, the preservability of the toner deteriorates.
On the other hand, when full color images are formed, yellow, magenta and cyan toners, optionally together with a black toner, are typically used. In order to produce full color images having good color reproducibility, the surface of the toner images has to be smoothed to some extent to decrease light scattering and therefore the viscoelasticity of the toners has to be decreased when the toners are melted. In this case, the color toners tend to cause the offset problem. In addition, when a release agent is included in color toners, the adhesion of the toner particles to each other is increased, and thereby the transferability of the toners is deteriorated. Therefore, it is difficult to use a fixing method for fixing color images, in which no oil or a small amount of oil is applied to a fixing roller.
Under such circumstances, the following toners have been proposed:    (1) a toner prepared by covering mother toner particles having a flow starting temperature not higher than 110° C. with small particles while embedding the small particles into the mother toner particles (Japanese patent No. 2,750,853);    (2) a toner prepared by covering a styrene-acrylic core material having a glass transition temperature of from 50 to 70° C. with a styrene based shell material having a higher molecular weight and a higher glass transition temperature (JOP 05-181301);    (3) a toner prepared by fixing a particulate resin on mother toner particles using a mechanical impacting method to reform the surface of the mother toner particles (JOP 06-342224);    (4) a toner prepared by microencapsulating a core material such as saturated fatty acids and saturated alcohols, which has a melting point of from 40 to 100° C. and which is suspended in water, with a particulate resin (JOP 08-254853);    (5) a toner prepared by overlaying a thermally stable layer and a thermoplastic resin layer having a Tg not lower than 65° C. on the surface of a particulate resin having a low viscosity (JOP 09-258480);    (6) a toner prepared by adhering a particulate resin having a Tg of from 60 to 110° C. on the surface of toner particles including a resin having a Tg of from 25 to 55° C. (JOP 2001-175025);    (7) a toner including a linear polyester resin having a softening point of from 90 to 120° C. and a carnauba wax (JOP 08-220808);    (8) a polymerized toner including a wax therein (JOP05-61242) and    (9) a toner prepared by extending or crosslinking an isocyanate-group-containing prepolymer in an aqueous medium using an amine (JOP 11-149180).
However, these toners do not necessarily have a good combination of low temperature fixability, offset resistance, preservability and transferability. Namely, the toners having a shell/core structure in which the shell is a uniform layer have poor low temperature fixability. The toner having a particulate shell has a low viscoelasticity when the toner is melted, and thereby the offset resistance is not satisfactory because the toner does not include a release agent. In general, wax tends to be mainly present on the surface of pulverized toners because the kneaded mixture tends to be fractured at interfaces between the wax and a resin. Therefore the pulverized toners tend to have poor transferability although having good offset resistance. In contrast, polymerized toners in which toner particles are prepared in an aqueous medium have poor offset resistance although having good transferability, because wax tends to be present inside of toner particles.
Because of these reasons, a need exists for a toner having a good combination of low temperature fixability, offset resistance, preservability and transferability even when a fixing method in which no oil or a small amount of oil is applied to a fixing roller is used.